The present invention relates to the suppression of interference on signal lines.
In the computer field, there are many situations in which signals have to be transmitted between different units of equipment. One common signalling technique uses DC signalling. This technique is commonly used where the distance between the two units of equipment is in the region of a meter up to around 1000 meters, and the bit rate is not excessively high.
This technique is largely free of interference if a single channel (of say 8 lines in parallel) is operated continuously. However, a problem can arise if the channel is not always in operation but the circuitry at the receiving end is left turned on. Interference can be generated on the channel, and this may be large enough to produce false triggering of the receiving circuitry.
Considering this in more detail, the receiving circuitry often has to be left turned on permanently, since it may not be known when the unit at the transmitting end will be in operation and it will not be feasible for the receiving circuitry to be turned on and off with the transmitting unit. A typical example is a microcomputer or workstation coupled to a host computer located elsewhere in a large building. The host computer has to keep its receiving circuitry permanently operating, while the various units to which it is coupled are powered up and powered down at unpredictable times.
The present signalling technique is effective at distances of up to around 1000 m. The signals are carried by a signal line which is referenced to a common return or ground line. The signal line is driven by a driver circuit, and the output impedance of such a driver circuit when powered own is typically high. The voltage on a signal line is therefore largely governed by the receiver circuit when the driver is powered down. However, variations in the local ground voltage between the two locations will be imposed on the return line at the driver end. Thus such local ground voltage fluctuations will appear between the signal and return lines at the receiver.
Also, it will often happen that the lines of two or more channels (between the same or different units) will run close to each other for substantial distances. There will then be substantial capacitive coupling between the lines of the different channels. Since the output impedance of a powered down driver is high, this can induce substantial voltages on the lines of a powered down channel. This cross-coupling will thus also appear at the receiver of a powered down channel.
There are various recognized standards for this type of transmission system. These specify, among other things, the sensitivity of the receivers. This sensitivity is such that interference (from either of these causes) can easily cause false triggering of the receivers of a powered down channel. This is clearly undesirable. Although the receiving unit can be designed to detect such false triggering, e.g. by detecting that it violates system protocols, this makes the system design more complicated, and incurs the overhead of processing these false signals (which are normally treated as attempts at logging into the system).